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I imagine the answer is unknown for the entire galaxy, but I would appreciate a notion of the scales and locations of regions where astronomers have ventured at least a guess to the distribution of organic matter in interstellar dust.
I am specifically looking for plots showing the distribution of some selected organic compounds (doesn't really matter which ones right now) within a given section of the sky. Or, if possible, within a given arc and range with respect to the galactic center (again, if possible).
The answer depends on which you are talking about (you mentioned both organic compounds and organic matter, which are two completely different things).
Organic compounds we do not have a clear definition of, but is most agreed upon to contain carbon atoms, either on their own (C), bonded with at least one hydrogen atom (C-H), or bonded with at least one other carbon atom (C-C). Compounds such as Methane (C-H4), Ethanol (C2-H6-O), and Glucose (C6-H12-O6) are textbook examples of organic compounds. Methane has been discovered on several objects within our solar system, including Mars, Venus, and Saturn. Methane has also been discovered on the exoplanet HD 189733b (though it is the only example we have of methane on an exoplanet), and often exists in interstellar clouds, showing that it is quite common throughout our galaxy.
Organic matter refers to matter composed of organic compounds, as part of an organism. While we are searching for organic matter as proof of extraterrestrial life, we have yet to discover any organic matter or clear evidence for such matter outside of our own planet.
NASA approves preliminary design of SPHEREx space telescopeAn artist’s impression of the Spectro-Photometer for the History of the Universe, Epoch of Reionisation and Ices Explorer, or SPHEREx, infrared space telescope. Image: NASA/JPL-Caltech
NASA managers have approved the preliminary design for the SPHEREx space telescope, a relatively low-cost infrared observatory designed to look for subtle differences in the distribution of matter across the cosmos that could provide evidence for inflation.
Built around a modest 20-centimetre (8-inch) telescope, the $242 million SPHEREx mission also will focus on the history of galaxy formation and look for water ice and organic compounds, both essential for life as it’s known on Earth, in stellar nurseries and in disks around stars where planets may be forming.
During its planned two-year mission, SPHEREx will collect data on more than 300 million galaxies and more than 100 million stars in the Milky Way, surveying the entire sky every six months to build up a three-dimensional map in 107 near-infrared colours.
“That’s like going from black-and-white images to colour,” said said Allen Farrington, the SPHEREx project manager at the Jet Propulsion Laboratory in Pasadena, California. “It’s like going from Kansas to Oz.”
Measuring the positions of galaxies relative to one another, astronomers may be able to discern statistical patterns caused by inflation, a hypothesised burst of ultra-fast expansion during the first instants of cosmic history.
SPHEREx stands for Spectro-Photometer for the History of the Universe, Epoch of Reionisation and Ices Explorer. It is expected to launch between June 2024 and April 2025.
NASA Finds Organic Salts Are Likely Present on Mars – Remnants of Ancient Martian Microbial Life?
This low-angle self-portrait of NASA’s Curiosity Mars rover shows the vehicle at the site from which it reached down to drill into a rock target called “Buckskin” on lower Mount Sharp. Credits: NASA/JPL-Caltech/MSSS
Salts Could Be Important Piece of Martian Organic Puzzle
A NASA team has found that organic, or carbon-containing, salts are likely present on Mars, with implications for the Red Planet’s past habitability.
A NASA team has found that organic salts are likely present on Mars. Like shards of ancient pottery, these salts are the chemical remnants of organic compounds, such as those previously detected by NASA’s Curiosity rover. Organic compounds and salts on Mars could have formed by geologic processes or be remnants of ancient microbial life.
Besides adding more evidence to the idea that there once was organic matter on Mars, directly detecting organic salts would also support modern-day Martian habitability, given that on Earth, some organisms can use organic salts, such as oxalates and acetates, for energy.
Curiosity’s Color View of Martian Dune After Crossing It: This look back at a dune that NASA’s Curiosity Mars rover drove across was taken by the rover’s Mast Camera (Mastcam) on February 9, 2014 – the 538th Martian day, or sol, of Curiosity’s mission. Credit: NASA/JPL-Caltech/MSSS
“If we determine that there are organic salts concentrated anywhere on Mars, we’ll want to investigate those regions further, and ideally drill deeper below the surface where organic matter could be better preserved,” said James M. T. Lewis, an organic geochemist who led the research, published on March 30, 2021, in the Journal of Geophysical Research: Planets. Lewis is based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Lewis’s lab experiments and analysis of data from the Sample Analysis at Mars (SAM), a portable chemistry lab inside Curiosity’s belly, indirectly point to the presence of organic salts. But directly identifying them on Mars is hard to do with instruments like SAM, which heats Martian soil and rocks to release gases that reveal the composition of these samples. The challenge is that heating organic salts produces only simple gases that could be released by other ingredients in Martian soil.
Mass spectrometers are a useful tool that NASA scientists rely on to find out if a sample contains certain molecules. Credit: NASA
However, Lewis and his team propose that another Curiosity instrument that uses a different technique to peer at Martian soil, the Chemistry and Mineralogy instrument, or CheMin for short, could detect certain organic salts if they are present in sufficient amounts. So far, CheMin has not detected organic salts.
Finding organic molecules, or their organic salt remnants, is essential in NASA’s search for life on other worlds. But this is a challenging task on the surface of Mars, where billions of years of radiation have erased or broken apart organic matter. Like an archeologist digging up pieces of pottery, Curiosity collects Martian soil and rocks, which may contain tiny chunks of organic compounds, and then SAM and other instruments identify their chemical structure.
Using data that Curiosity beams down to Earth, scientists like Lewis and his team try to piece together these broken organic pieces. Their goal is to infer what type of larger molecules they may once have belonged to and what those molecules could reveal about the ancient environment and potential biology on Mars.
“We’re trying to unravel billions of years of organic chemistry,” Lewis said, “and in that organic record there could be the ultimate prize: evidence that life once existed on the Red Planet.”
While some experts have predicted for decades that ancient organic compounds are preserved on Mars, it took experiments by Curiosity’s SAM to confirm this. For example, in 2018, NASA Goddard astrobiologist Jennifer L. Eigenbrode led an international team of Curiosity mission scientists who reported the detection of myriad molecules containing an essential element of life as we know it: carbon. Scientists identify most carbon-containing molecules as “organic.”
Since arriving at Mars in 2012, NASA’s Curiosity rover has drilled into rocks in search of organics – molecules containing carbon. Now, Curiosity has discovered ancient organics that have been preserved in rocks for billions of years. This finding helps scientists better understand the habitability of early Mars, and it paves the way for future missions to the Red Planet. Credit: NASA’s Goddard Space Flight Center/Dan Gallagher
“The fact that there’s organic matter preserved in 3-billion-year-old rocks, and we found it at the surface, is a very promising sign that we might be able to tap more information from better preserved samples below the surface,” Eigenbrode said. She worked with Lewis on this new study.
Analyzing Organic Salts in the Lab
Decades ago, scientists predicted that organic compounds on Mars could be breaking down into salts. These salts, they argued, would be more likely to persist on the Martian surface than big, complex molecules, such as the ones that are associated with the functioning of living things.
If there were organic salts present in Martian samples, Lewis and his team wanted to find out how getting heated in the SAM oven could affect what types of gases they would release. SAM works by heating samples to upwards of 1,800 degrees Fahrenheit (1,000 degrees Celsius). The heat breaks apart molecules, releasing some of them as gases. Different molecules release different gases at specific temperatures thus, by looking at which temperatures release which gases, scientists can infer what the sample is made of.
First Photograph Taken On Mars Surface: This is the first photograph ever taken on the surface of the planet Mars. It was obtained by Viking 1 just minutes after the spacecraft landed successfully early today [July 20, 1976]. Credit: NASA/JPL
Lewis analyzed a range of organic salts mixed with an inert silica powder to replicate a Martian rock. He also investigated the impact of adding perchlorates to the silica mixtures. Perchlorates are salts containing chlorine and oxygen, and they are common on Mars. Scientists have long worried that they could interfere with experiments seeking signs of organic matter.
Indeed, researchers found that perchlorates did interfere with their experiments, and they pinpointed how. But they also found that the results they collected from perchlorate-containing samples better matched SAM data than when perchlorates were absent, bolstering the likelihood that organic salts are present on Mars.
Additionally, Lewis and his team reported that organic salts could be detected by Curiosity’s instrument CheMin. To determine the composition of a sample, CheMin shoots X-rays at it and measures the angle at which the X-rays are diffracted toward the detector.
Curiosity’s SAM and CheMin teams will continue to search for signals of organic salts as the rover moves into a new region on Mount Sharp in Gale Crater.
Soon, scientists will also have an opportunity to study better-preserved soil below the Martian surface. The European Space Agency’s forthcoming ExoMars rover, which is equipped to drill down to 6.5 feet, or 2 meters, will carry a Goddard instrument that will analyze the chemistry of these deeper Martian layers. NASA’s Perseverance rover doesn’t have an instrument that can detect organic salts, but the rover is collecting samples for future return to Earth, where scientists can use sophisticated lab machines to look for organic compounds.
Reference: “Pyrolysis of Oxalate, Acetate, and Perchlorate Mixtures and the Implications for Organic Salts on Mars” by J. M. T. Lewis, J. L. Eigenbrode, G. M. Wong, A. C. McAdam, P. D. Archer, B. Sutter, M. Millan, R. H. Williams, M. Guzman, A. Das, E. B. Rampe, C. N. Achilles, H. B. Franz, S. Andrejkovičová, C. A. Knudson and P. R. Mahaffy, 30 March 2021, Journal of Geophysical Research: Planets.
First ‘Hot Molecular Core’ Discovered Outside Milky Way
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered the first ‘hot molecular core’ — a cocoon of complex molecules around a newborn star — in a galaxy other than our own.
This figure shows observations of the first hot molecular core to be found outside the Milky Way Galaxy with ALMA and a view of the region of sky in infrared light. Left: distributions of molecular line emission from a hot molecular core in the LMC observed with ALMA. Emissions from dust, sulfur dioxide (SO2), nitric oxide (NO), and formaldehyde (H2CO) are shown as examples. Right: an infrared image of the surrounding star-forming region (based on data from NASA’s Spitzer Space Telescope). Image credit: T. Shimonishi / Tohoku University / ALMA / ESO / NAOJ / NRAO.
“This is the first detection of an extragalactic hot molecular core, and it demonstrates the great capability of new generation telescopes to study astrochemical phenomena beyond the Milky Way,” said Dr. Takashi Shimonishi, an astronomer at Tohoku University, Japan.
“The observations have suggested that the chemical compositions of materials that form stars and planets are much more diverse than we expected.”
Dr. Shimonishi and co-authors have used ALMA’s resolving power to observe a young, massive star known as ST11 (full name 2MASS J05264658-6848469) in the Large Magellanic Cloud (LMC), a neighboring dwarf galaxy some 160,000 light-years away.
“LMC is an excellent target to study interstellar and circumstellar chemistry in different metallicity environments owing to its proximity and low metallicity (about one third of the solar neighborhood),” the astronomers explained.
“The low dust content in the galaxy results in harsh radiation environment, and thus photoprocessing of interstellar medium should be more effective in the LMC than in our Galaxy.”
“Furthermore, according to gamma-ray observations, the cosmic-ray density in the LMC is estimated to be lower than the Galactic typical values by a factor of four.”
“It is therefore highly anticipated that these environmental differences should affect chemical processes, and hot cores in the LMC should provide us key information to understand chemistry, particularly those of complex molecules, in low metallicity environments.”
Emission from a number of molecular gases in ST11 was detected. These indicated that the astronomers had discovered a concentrated region of hot and dense molecular gas.
This was evidence that they had found something never before seen outside of our Galaxy — a hot molecular core.
“Hot molecular cores are small, with a diameter of less than 0.3 light-years. These objects have a density over a thousand billion molecules per m 3 (far lower than the Earth’s atmosphere, but high for an interstellar environment) and warm in temperature, at over minus 173 degrees Celsius. This makes them at least 80 degrees Celsius warmer than a standard molecular cloud, despite being of similar density,” the scientists said.
“These hot cores form early on in the evolution of massive stars and they play a key role in the formation of complex chemicals in space.”
The ALMA observations revealed that this newly discovered core in the LMC has a very different composition to similar objects found in the Milky Way.
The most prominent chemical signatures in ST11 include familiar molecules such as sulfur dioxide, nitric oxide, and formaldehyde — alongside the ubiquitous dust. But several organic compounds, including methanol, had remarkably low abundance.
In contrast, cores in the Milky Way have been observed to contain a wide assortment of complex organic molecules, including methanol and ethanol.
The LMC has a low abundance of elements other than hydrogen or helium. The astronomers suggest that this very different galactic environment has affected the molecule-forming processes taking place surrounding ST11. This could account for the observed differences in chemical compositions.
The team’s findings were published in the Astrophysical Journal (arXiv.org preprint) on August 9, 2016.
Takashi Shimonishi et al. 2016. The Detection of a Hot Molecular Core in the Large Magellanic Cloud with ALMA. ApJ 827, 72 doi: 10.3847/0004-637X/827/1/72
Organic Compounds Found in Plumes of Saturn's Icy Moon Enceladus
They're similar to compounds on Earth that help to form amino acids.
Scientists have detected new types of organic compounds in the plumes that have been erupting from Saturn's icy moon Enceladus.
NASA's Cassini spacecraft collected invaluable data and images of Saturn and its moons over the approximately 20 years that the mission took place. While the mission ended on Sept. 15, 2017, with the craft diving toward the planet in a "Grand Finale," scientists continue to study the wealth of data that they gathered during the mission.
In one new study, scientists looked at the material that Enceladus ejects from its core using hydrothermal vents. The material mixes with water in the moon's subsurface ocean and is then emitted as water vapor and icy grains.
In studying these ejections, the team found organic molecules that are condensed onto these grains and which contain oxygen and nitrogen. This comes after the first discovery of organics on the moon in 2018.
Similar compounds on Earth take part in the chemical reactions that form amino acids, which are the organic compounds that combine to form proteins and are essential to life as we know it.
On Earth, energy, or heat, from hydrothermal vents on the ocean floor helps to fuel these amino acid-producing reactions. With these findings, scientists have suggested that perhaps something similar is happening on Enceladus and the hydrothermal vents under its subsurface ocean are aiding in the creation of amino acids on the moon.
"If the conditions are right, these molecules coming from the deep ocean of Enceladus could be on the same reaction pathway as we see here on Earth. We don't yet know if amino acids are needed for life beyond Earth, but finding the molecules that form amino acids is an important piece of the puzzle," Nozair Khawaja, who led the research team from the Free University of Berlin, said in a statement.
Now, the discovery of these organic compounds in no way equates to the discovery of life or even necessarily the building blocks of life. But it is another step in the direction of discovering whether or not amino acids might form on Enceladus and what that might mean with regard to the search for life in the universe.
"Here we are finding smaller and soluble organic building blocks &mdash potential precursors for amino acids and other ingredients required for life on Earth," co-author Jon Hillier said in the statement.
"This work shows that Enceladus' ocean has reactive building blocks in abundance, and it's another green light in the investigation of the habitability of Enceladus," co-author Frank Postberg added in the same statement.
To detect these compounds and come to this exciting conclusion, Khawaja's team used data from Cassini's Cosmic Dust Analyzer (CDA), which detected ice grains emitted in the moon's plumes and data from the CDA's spectrometer, which analyzed the composition of the grains.
These findings were published Oct. 2 in the journal the Monthly Notices of the Royal Astronomical Society.
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Mars’ Organic Carbon ‘Batteries’ Point to Biology –“Major Implications for Habitability”
“Revealing the processes by which organic carbon compounds form on Mars has been a matter of tremendous interest for understanding its potential for habitability,” said Carnegie’s Andrew Steele. “The discovery that natural systems can essentially form a small corrosion-powered battery that drives electrochemical reactions between minerals and surrounding liquid has major implications for the astrobiology field.” A similar process could occur anywhere that igneous rocks are surrounded by brines, including the subsurface oceans of Jupiter’s moon Europa, Saturn’s moon Enceladus, and even some environments here on Earth, particularly early in this planets’ history.
Carnegie’s Andrew Steele was a key member of the research team whose work on this project built off his discovery six years ago of indigenous organic carbon in 10 Martian meteorites. The organic molecules he found in 2012 are comparable to those found by Curiosity.
Like the meteoric samples, the rocks sampled by Curiosity must be heated by the rover’s instruments to very high temperatures, ranging between 500 and 800 degrees Celsius (932 and 1,472 degrees Fahrenheit), to have their organics released as gas. Because the hydrocarbons were released at such high temperatures, they may be coming from bigger, tough organic molecules within the rock.
Sedimentary rocks (mudstones) were drilled from four areas at the base of Mount Sharp, the central mound in Gale crater. Although the surface of Mars is inhospitable today, there is evidence that in the distant past, the Martian climate allowed the presence of liquid water–an essential ingredient for life–at the surface.
Analysis by Curiosity indicates that billions of years ago, a lake inside Gale crater held all the ingredients necessary for life, including chemical building blocks, energy sources, and liquid water. The mudstone gradually formed from silt that settled out of the water and accumulated at the bottom of the lake. Scientists estimated the age of the rocks by the crater count method. Since meteorite impact craters accumulate over time, the more craters a region has, the older it is. Although there was no way to directly date the organic material found within the rocks, it has to be at least as old as the rocks themselves.
The results indicate organic carbon concentrations on the order of 10 parts per million or more. This is close to the amount of observed in Martian meteorites and about 100 times greater than prior in-situ detections of organic carbon. Some of the molecules identified include thiophenes, benzene, toluene, and small carbon chains, such as propane or butene. Organic molecules containing chlorine were detected on Mars before.
Finding ancient carbon preserved right on the Martian surface gives scientists confidence that NASA’s Mars 2020 rover and the European Space Agency’s ExoMars rover will find even more organics, both on the surface and in the shallow subsurface.
“Are there signs of life on Mars?” asks Michael Meyer, NASA Program Scientist for the Mars Science Laboratory mission. “We don’t know but these results tell us we are on the right track.”
Steele says that the next steps must be looking for organic compounds that are released from the rock samples at lower temperatures.
Mars’ organic carbon may have originated from a series of electrochemical reactions between briny liquids and volcanic minerals, according to new analyses of three Martian meteorites from a team led by Carnegie’s Andrew Steele. The group’s analysis of a trio of Martian meteorites that fell to Earth—Tissint, Nakhla, and NWA 1950—showed that they contain an inventory of organic carbon that is remarkably consistent with the organic carbon compounds detected by the Mars Science Laboratory’s rover missions.
Steele’s 2012 team determined the organic carbon found in 10 Martian meteorites did indeed come from the Red Planet and was not due to contamination from Earth, but also that the organic carbon did not have a biological origin. This new work takes his research to the next step—trying to understand how Mars’ organic carbon was synthesized, if not by biology.
High-resolution Transmission Electron Micrograph (scale 50nm) of a grain from a Martian meteorite. Reminiscent of a long dinner fork, the organic carbon layers are found between the intact ‘tines.’ This texture is created when the volcanic minerals of the Martian rock interact with a salty brine and become the anode and cathode of a naturally occurring battery in a corrosion reaction. This reaction would then have enough energy–under certain conditions–to synthesize organic carbon. Credit: Andrew Steele
Organic molecules contain carbon and hydrogen, and sometimes include oxygen, nitrogen, sulfur, and other elements. Organic compounds are commonly associated with life, although they can be created by non-biological processes as well, which are referred to as abiotic organic chemistry.
He and his co-authors took a deep dive into the minerology of these three Martian meteorites. Using advanced microscopy and spectroscopy, they were able to determine that the meteorites’ organic compounds were likely created by electrochemical corrosion of minerals in Martian rocks by a surrounding salty liquid brine.
Ceres –The Organic Dwarf Planet: “It’s Like a Chemical Factory”
“With these findings, Ceres has gained a pivotal role in assessing the origin, evolution and distribution of organic species across the inner solar system,” said Southwest Research Institute scientist Simone Marchi. “One has to wonder about how this world may have driven organic chemistry pathways, and how these processes may have affected the make-up of larger planets like the Earth.”
“Ceres is like a chemical factory,” added Marchi. “Among inner solar system bodies, Ceres’ has a unique mineralogy, which appears to contain up to 20 percent carbon by mass in its near surface. Our analysis shows that carbon-rich compounds are intimately mixed with products of rock-water interactions, such as clays.”
A team led by Southwest Research Institute has concluded that the surface of dwarf planet Ceres is rich in organic matter. Data from NASA’s Dawn spacecraft indicate that Ceres’s surface may contain several times the concentration of carbon than is present in the most carbon-rich, primitive meteorites found on Earth.
Ceres is believed to have originated about 4.6 billion years ago at the dawn of our solar system. Dawn data previously revealed the presence of water and other volatiles, such as ammonium derived from ammonia, and now a high concentration of carbon. This chemistry suggests Ceres formed in a cold environment, perhaps outside the orbit of Jupiter. An ensuing shakeup in the orbits of the large planets would have pushed Ceres to its current location in the main asteroid belt, between the orbits of Mars and Jupiter.
Geophysical, compositional and collisional models based on Dawn data revealed that Ceres’ partially differentiated interior has been altered by fluid processes. Dawn’s Visible and Infrared Mapping Spectrometer has shown that the overall low albedo of Ceres” surface is a combination of rock-water interaction products such as phyllosilicates and carbonates and a significant amount of spectrally neutral darkening agents, such as an iron oxide called magnetite.
Because Dawn’s Gamma Ray and Neutron Detector limits magnetite to only a few percent by mass, the data point to the presence of an additional darkening agent, probably amorphous carbon, a carbon-rich organic material. Interestingly, specific organic compounds have also been detected near a 31-mile-wide impact crater named Ernutet, giving further support to the widespread presence of organics in Ceres’ shallow subsurface.
The new study also finds that 50-60 percent of Ceres’ upper crust may have a composition similar to primitive carbonaceous chondrite meteorites. This material is compatible with contamination from infalling carbonaceous asteroids, a possibility supported by Ceres’ battered surface.
“Our results imply that either Ceres’ accreted ultra-carbon-rich materials or that carbon was concentrated in its crust,” said said Marchi. “Both potential scenarios are important, because Ceres’ mineralogical composition indicates a global-scale event of rock-water alteration, which could provide conditions favorable to organic chemistry.”
Exploring Organic Environments in the Solar System (2007)
The search for understanding of how organic environments originated on the early Earth and throughout the universe and their association with life processes is the ultimate interdisciplinary field. Subdisciplines of the Earth sciences, biology, chemistry, astronomy, and the space sciences are all needed to contribute to the contemporary understanding of this complex problem.
The search for organics in the solar system involves a series of interrelated questions. These include where organic reservoirs are located in the solar system, 1 what specific organic compounds are present in these reservoirs, and what the location and identification of these organic compounds can tell us about both the evolution of the solar system and the possible presence of life at locations other than on Earth.
The discussion in Part II serves as an overview of all known inventories of organic compounds in the solar system, the possible means by which they were formed, and also&mdashbased on current observations&mdashlocations, not yet examined in detail, where organic compounds may be present. Recommendations for further exploration take into consideration the likelihood that significant organics will be found, the ease with which they can be found, and the anticipated amount and significance of data that can be accumulated from a particular research activity or spacecraft mission.
All the carbon in the universe is made by fusion reactions in stars. Carbon-12 ( 12 C) is created by the fusion of three helium-4 ( 4 He) nuclei. Carbon-13 ( 13 C) is made late in the lives of red giant stars, where formation of helium, catalyzed by 12 C, results in the formation of 13 C, 14 N, and 15 O via the CNO cycle outlined below:
For the purposes of this report, the interstellar medium is included in this search, both because Earth-based observations can be used to identify organics present there, and because it is thought that the interstellar medium was a source for organics present in the solar system&rsquos protoplanetary disk.
where e + , &gamma, and &nu are a positron, a gamma ray, and a neutrino, respectively. The 12 C/ 13 C ratio resulting from the CNO cycle is in the range of 15 to 20.
The CNO cycle proceeds in the shells of the red giant stars that contain high levels of carbon, where the conversion of 12 C to 13 C, and to 14 N and 15 O, proceeds after the bulk of the hydrogen has been converted to He. The pulsation of the red giant during this energetic process disperses the elements formed into the interstellar medium, where they serve as the starting materials for the formation of a new star.
The first organic compounds were formed from the carbon injected into the interstellar medium under the influence of cosmic rays and ultraviolet light. Simple hydrocarbons and other compounds that contain nitrogen, oxygen, and sulfur were formed in this cloud of dust and molecules. This process proceeded for about 10 7 years, producing additional organics before the dust cloud collapsed to form stars and their associated planetary systems.
In the solar system the evolution of carbon compounds proceeded during planetary system formation. The existing compounds were subjected to the shock waves resulting from the collapse of the dust cloud to stars and protoplanetary disks. The intense ultraviolet and x-rays emitted by the new star effected changes in some of the organics. Carbon compounds ultimately derived from the interstellar medium were accreted onto planetesimals in the early solar system, where considerable thermal and aqueous modification may have occurred. These planetesimals then aggregated to form planets, a process that further modified some of their organic constituents. The organics present in the atmospheres of the newly formed planets were subjected to solar ultraviolet radiation as well. Organics on and below the surface of planets were further changed by energy sources including heat from volcanoes, heating by transport into planetary interiors where they were subjected to heat and pressure, contact with hydrothermal systems that initiated reactions with water at high temperatures and pressure, and reduction by minerals. Volcanoes also injected volatile organics into the atmosphere where solar ultraviolet radiation and x-rays changed them.
2 Interstellar Chemistry
THE INTERSTELLAR MEDIUM
Inventory of Organic Compounds in the Interstellar Medium
Some 20 to 30 percent of the mass of our galaxy is in the form of the interstellar medium (ISM), i.e., the material between the stars. The ISM consists primarily of gas, with atomic or molecular hydrogen and helium contributing approximately two-thirds and one-third of the total mass, respectively. The next most abundant atoms, oxygen, carbon, and nitrogen, collectively account for about 1 percent of the ISM&rsquos mass. The remaining elements are present only in trace amounts. Approximately 1 percent of the mass of the ISM is in the form of micron-size dust particles. Astronomical observations, combined with studies of interstellar grains preserved in meteorites, suggest that the dust might consist variously of amorphous carbon, complex fullerenes, polycyclic aromatic hydrocarbons, diamond, silicon carbide, silicates, carbonates, and a host of other candidates, all with or without mantles of ices and/or organic compounds. 1
Important components of the ISM are molecular clouds, which are dense, massive objects found throughout the Milky Way and in many external galaxies. In these molecular clouds&mdashalso known as dense clouds&mdashthe gas density is 10 3 to 10 6 particles/cm 3 , which is very high by interstellar standards, and their masses can be as large as a million times the mass of the Sun. They are also usually very cold objects, with temperatures typically in the range from 10 to 100 K. Because of their high masses, these objects are the sites of star and planet formation, and also where complex gas-phase chemistry occurs.
Astronomical observations of the ISM have revealed the presence of numerous organic compounds. More than 125 different chemical species have been identified in interstellar and circumstellar regions, some containing 10 or more carbon atoms (Table 2.1). Assuming that the carbon in the ISM is present in cosmic abundance, then only 0.04 percent (by number) of the material there is carbon, even though approximately 80 percent of the observed species in the ISM are organic, including almost all of the larger molecules, many of which are relatively complex. 2 Organic compounds are, however, only a trace constituent of the ISM and account for less than 1 percent of its total mass. Inorganic compounds abound, with CO, for example, accounting for some 20 percent of the carbon in dense interstellar clouds. CO is, itself, outnumbered by the most common molecular species, H2, by a factor of approximately 10,000. The majority of the molecular species identified in the ISM have been discovered using high-resolution (1 part in 10 6 to 10 8 ) spectroscopic techniques of radio and millimeter astronomy. This
TABLE 2.1 Known Interstellar and Circumstellar Molecules
NOTE: The observations are of molecular emission by high-resolution spectroscopy with lines having aquality factor of 1 part in 10 6 to 10 8 . The identification of these molecules has been made on the basis of their pure rotational, rovibrational, or electronic spectra, which occur in the radio/millimeter, infrared, and optical/ultraviolet regions of the electromagnetic spectrum, respectively. The species are a composite of data obtained from avariety of astronomical sources, including comets, several dense interstellar clouds, and circumstellar envelopes.
aAlower-case &ldquoc&rdquo indicates a cyclic structure alower-case &ldquo1,&rdquo a linear structure and a &ldquo?,&rdquo a tentative identification.
SOURCE: Courtesy of H. Alwyn Wootten, National Radio Astronomy Observatory, available at http://www.cv.nrao.edu/
awootten/allmols.html, last accessed January 22, 2007.
triumph of radio astronomy has changed our perception of the universe as being predominantly a rarified atomic environment to one containing a large (organic) molecular component.
The Synthesis of Interstellar Molecules
The observed interstellar species listed in Table 2.1 are not those expected based on conditions of thermodynamic equilibrium. One indication of large deviations from equilibrium are the relatively large abundances of high-energy isomeric forms of species. For example, both HCN and its high-energy isomer HNC are observed with relative abundances such that [HNC]/[HCN] approaches or exceeds unity in some molecular clouds. At a canonical temperature of 20 K, the equilibrium abundance ratio is expected to be many orders of magnitude less than 1. The presence of many reactive free radicals and molecular ions in interstellar gas also indicates non-equilibrium conditions.
Further evidence that interstellar molecular abundances are not controlled by thermodynamics is found by consideration of the well-known reaction
At 20 K, the equilibrium constant for this process is calculated to be 10 500 molecules &minus 2 cm 6 . Given a typical interstellar gas density of 10 5 particles cm &minus3 , a complete conversion of CO to CH4 should occur in molecular clouds if chemical equilibrium prevailed. In contrast, CO is the second most abundant interstellar gas-phase molecule, so abundant, in fact, that it is used to map the distribution of molecular clouds in our galaxy and in external galaxies. Closely related to the high abundance of interstellar CO is the presence of polycarbon molecular species in unsaturated forms, in particular the long polyacetylene chains. These phenomena can all be explained by considering a chemical environment that is kinetically controlled, as opposed to thermodynamically controlled. The low-temperature, low-density conditions present in molecular clouds in fact favor a chemistry governed by kinetic effects.
The types of chemical reactions that can occur in interstellar clouds are limited by the physical environment in these objects. Although these regions are dense by interstellar standards, they are extremely rarified in comparison with conditions that can be obtained in terrestrial laboratories. This low density limits any chemical reaction, restricting it to a two-body process, whereas most reactions in the laboratory involve three bodies. These reactions have negligible activation energies because of the strong attraction between the positively charged ion and the neutral molecule. The energy released in the association of these two molecules drives the reaction at the low temperatures of the ISM. Processes with activation barriers generally will not occur within the lifetime of a molecular cloud (typically a million years).
One type of chemical reaction that fulfills interstellar criteria (i.e., two-body process, low activation energy barrier) are positive ion-molecule reactions of the general form
Because of the attractive force between a positive ion and a neutral species, these processes generally lack significant activation energies and have relatively fast rates, despite the fact that a third body is not participating to stabilize the products. The ion-molecule rate is also usually independent of temperature.
For ion-molecule reactions to occur, however, positive molecular ions must be present initially. The bulk volume of the dense clouds is not penetrated by starlight thus, the energy source for generating such ions is high-energy cosmic rays. Because the bulk composition of any given molecular cloud will be molecular hydrogen and atomic helium, cosmic-ray (100-MeV)-induced ionization produces primarily H2 + and He + cations. The secondary reactions then proceed via ion-molecule processes. One important process is the reaction of H2 + with H2, which occurs at the typical ion-molecule rate of k = 2 × 10 &minus9 cm 3 s &minus1 molecule &minus1 :
The reaction of He + with H2, in contrast, is too slow to be significant. If it were faster, it would immediately destroy the highly reactive He + and therefore essentially quench the formation of organic molecules.
CO, which is formed by ion-molecule reactions in molecular clouds, is present in relation to molecular hydrogen at a ratio of about 10 &ndash4 . Reactions with CO are also important in the ion-molecule scheme. CO reacts rapidly with H3 + and He + :
The product of the first reaction, HCO + , is extremely stable and is a major ion in dense molecular clouds. It has been used as a tracer of ionization and of course ion-molecule chemistry, given that, until very recently, H3 + had not been observed. (The detection of H3 + was eventually accomplished by infrared absorption spectroscopy, and the observed abundance in dense and diffuse clouds is in excellent agreement with theoretical calculations.) The second reaction is the primary basis for the very rich organic chemistry observed in interstellar clouds, because it leads to the production of C + . In this process, the He + formed by the cosmic-ray ionization of He generates a C + in a process 10 3 times faster than the direct cosmic-ray ionization of CO. This result is a direct consequence of the lack of reactivity of He + with H2.
C + is an important product because it can insert itself into other carbon-containing molecules to increase carbon chain length. For example, the typical synthesis for building larger organic molecules involves the reaction of hydrocarbon radicals such that:
followed by the addition of two hydrogens, leading to Cn+1H3 + . The neutral species is finally generated by dissociative electron recombination or by proton transfer to a suitable base. Such carbon insertion reactions most likely lead to the wide variety of carbon chains found in interstellar gas.
Ion-molecule radiative association becomes increasingly efficient with increasing molecular size. This type of process may also lead to the larger organic species. For example, methanol is thought to be created via the radiative association process
followed by dissociative electron recombination:
Other ion-molecule reactions can create even larger compounds. Methyl formate is synthesized from
The neutral species is then produced again by proton transfer to a suitable base or by dissociative recombination with an electron, creating HCOOCH3 and H. The main point is that, in principle, gas-phase ion-molecule reactions can create organic molecules in interstellar gas. Extensive chemical modeling of ion-molecule chemistry has been carried out and has been relatively successful in reproducing the abundances observed in interstellar space. It is not known, however, what degree of molecular complexity can be achieved through such reactions. This uncertainty remains one of the open questions for astrochemistry.
Although the bulk of the reactions in the ISM are initiated by cosmic rays or ultraviolet light, some reactions are believed to be initiated by neutral free radicals. For example, the amount of cyanoacetylene (HC3N) present is modeled more accurately by the addition of a cyano radical (CN . ) to acetylene than by ion-molecule reactions:
where the symbol . indicates a free radical.
It is worth noting that ion-molecule reactions are believed to be a route by which significant deuterium, and some 13 C, enhancement occurs in organic molecules in the ISM. This fractionation effect arises from small differences in the molecular binding energies. 3 For example, the ratio of deuterated isotopomers to their normal counterparts may be enhanced by up to four orders of magnitude compared to elemental deuterium/hydrogen abundances in the ISM. 4
Surface Reactions in the Interstellar Medium
Reactions of compounds on dust grains are another source of organic compounds in the ISM. The dust grains are produced in the circumstellar shells of red giant stars that condense from the hot material (mainly silicates) emitted from the stellar surface. Carbon stars eject carbon and partially hydrogenated carbon into the ISM by this route. Stellar ejecta from supernovas are also believed to be a source of grains. A mantle of ice consisting of water, CO, CO2, and organics condenses on these grains at the low temperatures (
10 K) present in the dense clouds (Table 2.2).
Ultraviolet light in the ISM initiates reactions that lead to the formation of more complex structures from the simpler compounds in the mantle on the grains. One source of the ultraviolet is the radiation emitted by molecular hydrogen, following collisional excitation by electrons produced by cosmic-ray ionization. 5 The 100- to 200-nm-wavelength light has an average flux of 10 3 photons cm &minus2 s &minus1 that is about 10 &minus5 that of the ultraviolet flux in the diffuse ISM. Stars are a second source of ultraviolet light that is impinging on the dust in the outer regions of the dark ISM. A third source is the ultraviolet emissions from young stellar objects (newly formed stars) in the dark ISM that irradiate the dust in their vicinity. The radiation not only initiates chemical reactions but also causes the evaporation of the icy mantles from the dust grains.
The ultraviolet processing proceeds by dissociating the molecules in the mantle into free radicals. These reaction intermediates are stable at 10 K, but if the grain is warmed by absorption of additional ultraviolet photons, the radicals move around in the mantle and react with the other molecules present. If the reaction is exothermic, the
TABLE 2.2 An Inventory of Interstellar Ices Based on Infrared Spectroscopy
NOTE: Abundances are expressed as percentages of the H2O abundance for three categories of sight-line. A range of values generally indicates real spatial variation where followed by a colon, it may merely reflect observational uncertainty. Values for XCN, an unidentified molecule containing C&equivN bonds, are based on an assumed band strength (D.C.B. Whittet, P.A. Gerakines, J.H. Hough, and S.S. Shenoy, &ldquoInterstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust Near the Diffuse/Dense Cloud Interface,&rdquo Astrophysical Journal 547(1): 872-884, 2001). A dash indicates that no data are currently available. SOURCE: After D.C.B. Whittet, Dust in the Galactic Environment, 2nd Edition, Series in Astronomy and Astrophysics, Institute of Physics Publishing, London, U.K., 1992.
energy released may evaporate the grain mantle, thus ejecting the compounds present into space. Mantles may also be heated and evaporated by collisions with other grains and the shock waves from supernovas.
The presence of silicates in the grains together with water, CO, and CO2 in their mantles, with smaller amounts of methanol, formaldehyde, formic acid, and methane, has been detected by infrared spectral studies. 6 , 7
The C-H stretching frequency of organics is visible in absorption bands at 3.4 µm characteristic of CH3 and CH2 groups. The similarities between the aliphatic C-H stretch region as seen in spectra of dust clouds in our own galaxy and as seen in the spectra of more distant galaxies suggest that the organic component of the dust in the ISM is widespread and may be an important universal residue of abiotic carbon.
The formation of molecular hydrogen from hydrogen atoms cannot occur by binary gas-phase reactions. However, hydrogen atoms have high surface mobility on the grain mantle and can readily combine on a surface to create H2. The energy released in the process of H2 formation is sufficient to desorb the molecule from the grain surface, even at 10 K.
The reduction of CO to formaldehyde and methanol is unlikely to occur by the action of cosmic rays because of the high activation energy required for the reactions. 8 In addition, some researchers have suggested that the amount of methanol in the grain mantles (5 to 10 percent of the water present) is much greater than that in the gas phase, suggesting that the mantle methanol was formed in the solid phase, 9 , 10 a claim that is in conflict with the previously proposed synthesis of methanol in the gas phase by radiative association (see above). It is likely that methanol is formed by both processes. It is possible that the reduction of CO in the mantle by hydrogen atoms is the source of the formaldehyde and methanol. The reaction with hydrogen atoms has no activation energy because hydrogen atoms can tunnel through the activation barriers on the mantle surface.
Extensive laboratory studies have shown that the ultraviolet irradiation of simulated grain mantles results in the generation of more complex organics. 11 - 14 Unfortunately, the laboratory studies are by necessity carried out with a high ultraviolet flux and thus are not representative of interstellar conditions. Since the precise composition of the grain mantles is not known, it is not possible to accurately extrapolate from the laboratory simulations to the amounts of these compounds in the dark ISM. For example, the higher yields of amino acids formed in the experiments of Munoz Caro et al. 15 probably reflect the use of a 10-fold lower ratio of water to the other reactants than was used in the comparable study by Bernstein et al. 16
It is difficult to compare the extent of formation of organics in the ISM by comparison of the products formed by cosmic rays and ultraviolet. The bulk of the compounds listed in Table 2.1 were formed in gas-phase reactions driven by cosmic rays. The presence of these compounds in the ISM was determined by high-resolution radio astronomy, a technique that is very sensitive and also makes it possible to determine the structures of the compounds. Infrared spectroscopy is much less sensitive than radio astronomy and is a technique that provides information about the functional groups in the organics and not an exact structure when a mixture of compounds is present. The different characteristics of the two spectral measurements make it difficult to compare the amounts and the diversity of compounds formed.
A large number of modeling programs exist to predict gas-phase reactions and molecular abundances. The extension of the general chemical modeling programs to surface chemistry faces a number of problems and uncertainties not encountered with gas-phase binary reactions. The variables in the kinetic models are generally the densities of the reactants in the gas phase. The incorporation of surface reactions with known gas-phase reactions into a master reaction scheme presents some significant difficulties. There is a large asymmetry in the fundamental understanding of binary (gas-phase) encounters and processes on surfaces. Several problems require experimental and theoretical resolution before a quantitative model, such as discussed for the gas-phase chemistry ion-molecule chemistry, will be obtained for surface reactions. The problems include the following:
The size and surface area of the grains as well as the chemical composition are not well characterized. It is difficult to model a surface of unknown composition.
The exact mechanisms for reactions on surfaces are not well characterized. Also, it is difficult to find desorption processes for reaction products that are effective at low temperatures (10 K).
The usual gas-phase reaction rate theory is not applicable to gas&ndashsurface reactions. Probability theory must be applied, and hence there are no exact solutions.
Broad Interstellar Features and the Organic Inventory
Broad emission features have been routinely observed in molecular clouds at infrared wavelengths, using spectroscopic techniques. The origin of these features, known as unidentified infrared bands, are most likely emissions from polycyclic aromatic hydrocarbons (PAHs). They occur at wavelengths that are suggestive of both the aliphatic and aromatic C-H stretching frequencies, as well as C-H deformation modes. These data indicate that organic material is present in interstellar gas that consists of large unsaturated hydrocarbons. Indeed, as already mentioned in the previous section, the spectral similarities between the aliphatic C-H stretch feature seen in interstellar dust in our galaxy and corresponding features seen in the spectra of more distant galaxies suggest that the organic component of the dust may represent an important universal residue of abiotic carbon. 17
So-called diffuse interstellar bands&mdashi.e., spectral features arising due to the absorption of visible light&mdashare also observed. Hundreds of these bands have been observed, but none have been assigned to specific compounds. Currently PAHs appear to be the most likely structures absorbing the visible light, but this assignment remains to be verified.
Carbon stars are the sources proposed for the presence of PAHs in the ISM. The emission spectra of interstellar dust clouds indicate that PAHs are widespread but contribute only 5 to 10 percent of the total carbon. They have been found in interstellar dust grains, in unequilibrated chondrites, and in the martian meteorite ALH84001.
Proposed Research on Organics in the Interstellar Medium
Ion-molecule reactions are the fastest gas-phase processes known. Their properties make them prime candidates for producing interstellar molecules. Consequently, understanding these reactions is essential for evaluating the chemistry of the ISM, especially considering that this is a low-temperature environment. Reaction rates are not known for many ion-molecule, radiative association, and even certain neutral-neutral reactions that involve rather abundant interstellar carbon-bearing species. Nor are many of the branching ratios known for the products of dissociative electron recombination reactions, the main mechanism by which neutral organic species are produced. How material formed initially in dense, interstellar molecular clouds and evolved through star formation and subsequent nebular condensation is at present highly speculative. Reaction rates should be measured experimentally in the laboratory, especially at low temperatures. Also, theoretical calculations of reaction rates and reaction potential surfaces would be helpful for those processes that are too difficult to be determined by experiment, or for comparison with the experimental results. These data will enable models of interstellar chemistry to be more accurate in the calculation of abundances and in the prediction of possible new organic species. Such data will also help elucidate the major reaction pathways for the production of carbon-bearing molecules. In the laboratory, high-resolution infrared spectral measurements, including pure vibrational and rovibrational studies, of possible organic molecules will suggest other possible interstellar organics. Investigations are needed of carbon-bearing radicals and ions that might function as reaction intermediates in interstellar processes. The data obtained will enable astronomers to study additional carbon-bearing compounds in the ISM and therefore complete the inventory of organic material outside the solar system. The laboratory investigations thus should be followed up with the appropriate astronomical studies, using available telescope facilities, both ground-based and future air- and space-borne platforms such as the Stratospheric Observatory for Infrared Astronomy (SOFIA) and Herschel, as well as the Spitzer Space Telescope. Laboratory work includes both gas-phase and solid-state experiments. Astronomers additionally need to establish more complete databases for organic compounds in interstellar objects. Currently, molecular abundances are known for only a small subset of sources, and often only one such object. Hence, it is currently impossible to evaluate the diversity of organics in the ISM. Systematic observations of the key organic compounds in a statistical sample of molecular sources will be helpful in this regard.
The early evolution of a young stellar object proceeds with rapid and dramatic changes. 18 Stars begin their lives in molecular clouds. As the cloud starts to fragment and collapse, a dense opaque protostellar core forms,
typically a few thousand to 10,000 astronomical units (AU) across, and this core falls inward, supplying material (dust, gas, ices&mdashincluding a rich array of organic molecules) to a central star. Because it is difficult to remove angular momentum from the gas during infall, the material accretes onto a rotationally supported disk surrounding the protostar. Dozens of these protoplanetary disks have been observed.
This main accretion phase is often simultaneously accompanied by prominent outflows of material (jets). When the star has accreted approximately 90 percent of its final mass, it will become a pre-main sequence star, just below the mass/temperature limit for hydrogen fusion. The disks can be up to a few hundred AU across with low densities (10 6 particles cm &minus3 ) in the outer region and with densities increasing to 10 9 particles cm &minus3 near 100 AU. Temperatures remain low (
10 K) at these distances but increase close to the central protostar.
The evolution of the core and formation of the disk around the star are only broadly understood and not yet well constrained by observations. The evolution of high-mass stellar systems and low-mass systems proceeds somewhat differently. Less is known about high-mass star formation because most of the formation phase occurs while the star is embedded in an optically thick cloud of material and is therefore unobservable. One epoch in the formation of high-mass stars that has been observed is the so-called hot-core phase. In this transition phase before a hot massive young stellar object ionizes its surroundings, the object just begins to heat the surrounding neutral gases and can vaporize grain mantles.
A hot-core phase can also occur during the formation of low-mass stars like the Sun, if these stars are formed in the proximity of a massive star. The radiation from the massive star will vaporize the icy grain mantles of the small protostar and generate a hot core. The volatiles released from the hot core of a small or massive star will be subjected to the ultraviolet radiation that drives the formation of more complex organics from the volatiles released from the icy mantles.
The study of the chemical processes taking place in protoplanetary disks is limited by the angular resolution of submillimeter instrumentation, although the construction of larger submillimeter telescopes and more sensitive arrays, such as Atacama Large Millimeter Array (ALMA) in Chile, will help tremendously at these wavelengths. In addition, the Spitzer Space Telescope, SOFIA, and the James Webb Space Telescope will help in the mid and far-infrared. Numerical simulations and chemical models are able, in combination with observations, to help examine the chemistry in the disks, although there may be differences in the chemical processes for high- and low-mass objects. The chemistry and chemical processes in young stellar objects may be considered in several different regimes as shown in Table 2.3.
TABLE 2.3 The Chemistry and Chemical Processes in Young Stellar Objects
Components of the Protoplanetary Disk
Molecules Detected in Millimeter/Submillimeter Wavelength Region
Molecules in the Infrared Wavelength Region
Principal Chemical Processes Believed to Occur
Formation Stage of the Protoplanetary Disk
Molecular ions, carbon chains
Cloud fragmentation collapse to protostar
Cold envelope around protostar
Enters main accretion phase (class 0)
Sublimation, gas-phase reactions
Protostar and disk accretion (class I)
Shocks, sputtering, photodissociation, ionization
Protostar accretion, bipolar outflows, envelope dissipation (class II)
SOURCE: Data from E.F. Van Dishoeck and F.F.S. van der Tak, &ldquoChemistry in Envelopes Around Massive Young Stars,&rdquo pp. 97-112 in Astrochemistry: From Molecular Clouds to Planetary Systems, IAU Symposium 197 (Y.C. Minh and E.F. van Dishoeck, eds.), Astronomical Society of the Pacific, San Francisco, Calif., 2000.
Low-Temperature Chemical Processes in Protoplanetary Disks
In the cold regions of the precursor molecular cloud, the chemistry is dominated by ion-molecule reactions, resulting in small radicals and unsaturated molecules. As the disk warms up, molecules are released from the grains via sublimation, and this initiates gas-phase reactions, which can produce molecules such as H2CO, C2H2, CH3OH, and others. 19 These processes can lead to deuterium fractionation in the disk, so that the high deuterium/hydrogen ratios seen in comets may not necessarily imply preservation of interstellar material. Numerical simulations and chemical models will benefit greatly from high spectral and spatial observations made from new interferometric millimeter arrays (e.g., ALMA). This new observing tool will enable an understanding of potential chemical gradients in the disks in planet-forming zones. It should be noted that, once beyond the cold-core stage, the chemistry of the protostellar disk cannot be understood outside the context of a specific dynamical disk model.
High-Energy Processes in Protoplanetary Disks
Dynamic magnetic fields in young stellar objects can lead to violent reconnection phenomena (analogous to solar flares) that accelerate particles to high energies (MeV to GeV i.e., similar to cosmic-ray energies), which can heat gases to x-ray temperatures. This process has been observationally detected from satellites with x-ray detectors and from nonthermal radio continuum radiation. In addition to heating, the x-rays cause ionization and excitation of molecules. The secondary electrons from the ionization can produce molecular ions such as H3 + and HeH + . Millimeter emissions from CO, HCN, CN, and HCO + have been seen around young stellar objects and attributed to x-ray-induced chemistry.
The x-ray ionization dominates the ionization caused by external cosmic rays out to about 100 to 1000 AU. High-spatial-resolution spectra can distinguish the mechanism inducing the chemistry because the cosmic-ray and x-ray ionizations operate on different spatial scales in the disks. In addition, x-ray irradiation of disk dust grains, which may contain a variety of carbonaceous compounds (such as PAHs, aliphatic hydrocarbons, and so on) can result in dehydrogenation and breaking of aromatic rings. External cosmic-ray irradiation does not play much of a role in the dense inner disk, but beyond 10 AU, where the density becomes low enough that the rays are not completely attenuated, cosmic-ray irradiation produces H3 + and He + ions that can convert CO and H2 to CO2, CH4, NH3, and HCN.
Shock Waves in Protoplanetary Disks
Shock waves occur in protoplanetary disks where the outflow from the protostar collides with the surrounding cloud material, and there are also accretion shocks from the material infalling onto the disk. The accretion shocks occur where the material rains down on the disk at speeds greater than the local speed of sound. The shock waves compress and heat the gas and can therefore affect the chemistry in the disk. In high-speed shocks (producing abrupt discontinuities in the conditions in the gases), temperatures can reach 10 4 to 10 5 K, and molecules will dissociate. These can reform in the warm wake of the shock. Ices can recondense as an amorphous solid on cold grains, and this process will result in enhanced volatile trapping in the ices. Thus a mixture of unaltered and modified grains in the disk can result. In lower-speed shocks, temperatures are not as high (
10 3 K), and endothermic reactions can produce new species not usually seen in the ambient medium. In addition, in the low-velocity shocks, ices can be removed from grain mantles as grains are driven through the medium via sputtering.
Theoretical modeling of protoplanetary disks is in its infancy, and much more research is required in order to predict the chemical reactions of the interstellar molecules when subjected to the energy sources associated with the process of star and planet formation. Laboratory research is also needed on ion-molecule chemistry, photochemistry, and reactions in shock-heated gases that model the changing conditions in solar system formation. These interdisciplinary studies may best be carried out in collaborative efforts involving planetary scientists, astronomers, and chemists.
INTERPLANETARY AND INTERSTELLAR DUST
Interplanetary dust particles (IDPs), which are formed by impacts between asteroids and by the sublimation of cometary materials, provide an opportunity to study extraterrestrial organic chemistry. Multiple flights to Earth&rsquos stratosphere (at altitudes of
20 km via the use of U2 aircraft) have yielded a relatively large collection of IDPs ranging in size from
5 to 50 µm. Given the small size of these particles, efficient radiative heat transfer successfully offsets the frictional heating developed during atmospheric entry, thus minimizing thermal alteration of both inorganic and organic phases and preserving what may be the most pristine extraterrestrial organic matter accessible for study on Earth. The integrated flux has been estimated at 4 × 10 10 gC/yr.
IDPs are classified as anhydrous or hydrous. Anhydrous IDPs contain silicate glass and minerals such as pyroxene and olivine. A cometary origin is likely 21 and would have protected these particles from hydrothermal processing on parent bodies. Consequently, they may be the most primitive material available for study on Earth. Hydrous IDPs are dominated by clays and probably derive from asteroids. 22 Both types of IDPs contain carbon at abundances ranging up to 90 percent by weight. 23 , 24
Analysis of this carbon&mdasheven determining whether it is an oxide, organic matter, or graphite&mdashis difficult because of the small particle size. This problem has been partly circumvented by use of Fourier transform infrared (FTIR) microspectrophotometry, analytical transmission electron microscopy (TEM with electron energy loss spectroscopy to derive chemical information), and scanning transition x-ray microscopy (STXM), the last utilizing synchrotron-based soft x-ray sources in order to examine the carbon-1s absorption edge. These techniques have shown that the organic matter in both anhydrous and hydrous IDPs is similar in that both types include aromatic and aliphatic carbon skeletons as well as ketones and carboxylic acids. 25 , 26 In comparison with extraterrestrial organic matter in carbonaceous chondrites, the organic matter in IDPs is, in general, less aromatic and more oxidized (predominantly as carboxyl groups). Studies employing an ion probe 27 as well as analytical TEM 28 reveal that the abundance of nitrogen in IDP organic matter is considerably greater than that observed in organic residues from carbonaceous chondrites.
One of the more intriguing aspects of organic matter in IDPs, revealed by the recent development of powerful microscopic analyses, 29 - 32 is the level of microscopic heterogeneity, in terms of both organic structure and isotopic abundances (e.g., H/D and 14 N/ 15 N). Significant variation in aromatic, aliphatic, and carboxyl concentrations has been revealed using analytical TEM and STXM 33 , 34 (Note that micro-FTIR lacks the spatial resolution to reveal such spatial heterogeneity in functional group distribution.) Enormous hydrogen and nitrogen stable isotopic anomalies ( 2 H and 15 N) have also been observed. Hydrogen isotopic anomalies have been correlated with organic-rich domains. 35 Work by Keller et al. 36 concludes that aliphatic carbon is the dominant carrier of deuterium. Moreover, high-deuterium anomalies have also been correlated with organic-poor regions of IDPs, i.e., hydrated silicates. 37 Nitrogen isotopic anomalies correlate spatially with the organic phases, 38 and recent analytical TEM reveals that this nitrogen is likely an amine, possibly a substituent on aromatic moieties.
Some anhydrous IDPs have revealed localized deuterium anomalies (measured relative to hydrogen and normalized to the deuterium:hydrogen ratio in Earth&rsquos oceans) as high as 11,000&permil (1&permil is 1 part in 1,000), 39 and, in extreme cases, entire IDPs record bulk deuterium anomalies as high as
25,000&permil. Such high deuterium contents are probably derived from molecular-cloud material. The record of solar system evolution encoded by organic matter in IDPs may therefore exceed that recorded by the organic constituents of carbonaceous chondrites. Most of the advances in IDP research have occurred in the past several years. In the near future, the application of new technologies&mdashe.g., the nanoscale secondary ion mass spectrometer (NanoSIMS)&mdashand advances in synchrotron-based instrumentation are likely to yield further, highly significant results.
Interstellar (as opposed to interplanetary) dust grains are less well characterized. While the latter are believed to have formed in the solar system, the former formed via condensation in circumstellar regions around evolved stars, including red giants, carbon stars, asymptotic giant branch stars, novas, and supernovas. Information on the nature of interstellar grains is available from two sources, astronomical observations and laboratory studies of meteorites. The astronomical evidence is derived primarily from observations of the infrared emissions from the dust itself or studies of the dust&rsquos ability to scatter, polarize, or redden the light of background stars (the so-called interstellar extinction). Laboratory studies of meteorites have revealed nanoparticles of, for example, diamond